Two techniques in common usage for determining total organic carbon (TOC) in aqueous solutions are the combustion method and the photooxidation method. In both, organic matter is oxidized to carbon dioxide which is subsequently measured by standard techniques such as infrared, conductivity, gas chromatography or flame ionization detection after reduction to methane. The combustion method is capable of rapid analysis on small samples (100 .mu.L to 200 .mu.L), however, it is not ideally suited for the accurate measurement of low TOC concentrations (less than 1 ppm). The photooxidation method is better suited to the determination of lower TOC concentrations than the combustion method since much larger samples can be used (up to 100 mL).
In the photooxidation method ultraviolet light of 180 to 260 nm wavelength or shorter is commonly used. Low pressure mercury vapour lamps have a major emission at 184.9 and medium pressure mercury lamps have a major emission at 253.7 nm. Both types of lamp may be used to effect photochemical oxidations (Danny M. Mar U.S. Pat. No. 4,273,558). Persulfate ions are frequently added to promote the oxidation (Janice L. Haverty, Ultrapure Water, Sep./Oct. 1984). Regardless of which ultraviolet lamp is used it is essential to use a quartz envelope around the lamp and between the lamp and the solution in order to transmit a maximum amount of ultraviolet light.
The photo-induced oxidation is believed to involve hydroxyl radicals in the case of the 184.9 nm light and sulfate radicals when persulfate photooxidation is used. Sulfate radicals are generally less effective than hydroxyl radicals in the oxidation of organics (P. Neta and R. E. Huie, J. Phys. Chem. 90, 4644 [1986]) but react with water to give hydroxy radicals (R. W. Matthews, H. A. Mahlman, and T. J. Sworski, J. Phys. Chem. 76, 1265 [1972]). Whether the photooxidation proceeds via hydroxyl radicals or sulfate radicals, the rate is enhanced by the addition of persulfate presumably because of the greater yield of photolytically generated oxidizing radicals. In any event one of the main disadvantages of the photooxidation method is that different organic compounds very often have different rates of photooxidation unless the relative ratios and the nature of organic compounds within a given aqueous solution are known, it is difficult to accurately compare the yield of carbon dioxide liberated therefrom as a result of photooxidation, with a calibration curve of the yield of carbon dioxide liberated from an aqueous solution of known organic compounds of different relative ratios and/or which are different from the aforementioned organic compounds.
A need exists for a quick, simple and economical method of detecting the presence of an organic compound in water or an aqueous composition for a variety of applications. For example formalin solutions are used to disinfect kidney dialysis machines after change of filters. After rinsing with water it must be ensured that no trace of formaldehyde remains in the machine and thus it is necessary to analyse the water used for the final rinse.
The present inventor has found that photocatalytic oxidation over thin films of titanium dioxide illuminated with near ultraviolet light is a powerful method of oxidizing dissolved organic solutes in aqueous solution. The ambient oxygen dissolved in the aqueous solution is the oxidizing agent and the photogenerated holes or hydroxyl radicals are the photocatalytic reactive intermediates. The reaction appears to be quite general for all organics in solution. A number of organic compounds have been shown to be totally mineralized (that is, converted to inorganic products) and it seems likely that this will occur with most organic compounds. Since photocatalytic oxidation is a fast, simple and inexpensive technique it would be advantageous if the technique could be utilized to detect the presence of an organic compound in water or an aqueous composition.
It is an object of this invention to provide a method and system for qualitatively and quantitatively determining organic matter in an aqueous solution including an oxidizing agent.